associated himself with the English and Scottish settlers in his vicinity, and as lieutenant-colonel to Sir Ralph Gore took an active part against the Irish rebels. A somewhat verbose and egotistical account of his action was on 4 June 1642 addressed by Mervyn to the speaker of the House of Commons in London. By order of that house it was printed at London and sold at ‘the Irish warehouse at Stationers' Hall,’ and entitled ‘An Exact Relation of all such Occurrences as have happened in the several counties of Donegall, Londonderry, Tyrone, and Fermanagh, in the North of Ireland, since the beginning of the Rebellion.’ Mervyn received a commission as colonel of one of the British regiments in Ulster, and with others of these commanders waited on the parliament at London in 1643 and on Charles I at Oxford for the purpose of obtaining money and supplies. On 5 July 1643 Mervyn was examined before a select committee of the House of Commons in London, mainly in reference to his relations with Colonel Rory Maguire. This examination, attested by Mervyn's signature, was published with other papers at London in 1643 by order of the House of Commons.
The zeal which Mervyn displayed against the covenant induced Ormonde in 1644 to appoint him governor of the town of Derry. Mervyn, however, soon afterwards took the covenant and excused his conduct to Ormonde on the ground of expediency. Mervyn was accordingly removed from the government of Derry, but continued as a ‘British colonel’ to command his regiment in its vicinity.
Towards the close of 1648 Mervyn was taken ‘insidiously’ by parliamentarians in Ulster, and by order of George Monck, afterwards first duke of Albemarle, then in command there, he was sent to London as a prisoner. He appears not to have been long detained, and in June 1649 he co-operated with Sir Robert Stewart against Sir Charles Coote, president of Connaught under the parliament. Coote, in a letter to the council of state at London on 15 Aug. 1649, charged Mervyn with having forged articles purporting to have been authorised by him. Later in the same month Ormonde employed Mervyn and the Bishop of Raphoe to confer with Owen O'Neill, general of the Irish in Ulster, on matters of importance to the interests of Charles II. Influenced probably by the recent arrival of Oliver Cromwell in Ireland with forces of the parliament of England, Mervyn withdrew from the royalist party and came to an arrangement with Coote at Derry.
In June 1658 Mervyn obtained formal admission to the society of King's Inns at Dublin, then under the control of law officers of the Cromwellian government. He soon after took part with Sir Charles Coote and Lord Broghill in the movements in Ireland for the restoration of Charles II. In conjunction with them and their associates Mervyn framed the instrument adopted by the king in reference to legal arrangements for Ireland. Mervyn was knighted, and on 20 Sept. 1660 he received the appointment of chief serjeant-at-law in Ireland. In February 1660–1 he was named as a commissioner for executing the king's declaration from Breda concerning Ireland. He was also made a commissioner to ascertain the value of lands in Ireland let out to ‘adventurers’ and soldiers, and a trustee for the officers who had served the king before June 1649. On 8 May 1661 Mervyn was chosen speaker of the House of Commons, Dublin, and on his presentation on the 11th of that month in the House of Peers there, he delivered an elaborate and pedantic oration, which was printed. In the same year Mervyn was included among the commissioners elected by parliament to proceed to England concerning arrangements for the settlement of Ireland, and Sir John Temple was appointed to act as deputy-speaker. Mervyn resumed his place at Dublin as speaker in May 1662, and in July of that year delivered a congratulatory address to the Duke of Ormonde, in the presence chamber at Dublin Castle, which was subsequently published. In February 1662–3 Mervyn, on behalf of the House of Commons, Dublin, presented to Ormonde, as viceroy, a series of rules and directions which they proposed should be put in operation in the execution of the act of settlement with a view to promote and secure the interests of protestants in Ireland. These propositions and the mode of Mervyn's advocacy of them were distasteful to the king's advisers in England, and a royal letter was addressed to Ormonde with a grave censure of the proceedings.
Mervyn continued to act as speaker till the termination of the parliament in 1666. In his official capacity as a commissioner in connection with lands he was reputed to have been influenced by pecuniary considerations. He was also suspected of having been connected with a plot against the government in which some members of parliament were believed to be implicated. He died at Dublin on 24 Oct. 1675, and was interred in the church of St. Werburgh in that city.
A sum of 6,000l. which Mervyn claimed as due to him for his ‘long and faithful service to the protestant interest in Ireland’
